(1) Field of the Invention
The present invention relates to an optical beam scanning apparatus which scans an intensity-modulated optical beam over a target surface with an optical scanning device. The intensity-modulated optical beam is produced by driving a light emission device such as a semiconductor laser.
(2) Description of the Related Art
In an electrophotographic apparatus, an optical beam scanning device has been used for scanning a beam that is by image information.
In such an optical beam scanning apparatus, it is required to keep the amount of light emitted from the light emission device, such as a light emission diode (LED), to a predetermined level.
Namely, in such a light emission device, the intensity of the optical scanning beam changes depending on the conditions of the optical scanning device and the ambient temperature.
Therefore, the gray level (concentration) becomes uneven in an image formed in such an electrophotographic apparatus described above.
For this purpose, a current for driving a light emission device has been set to make constant a light intensity of the optical beam based on a receiving output of a monitor diode, which monitors the light intensity of the optical beam from the light emission device. However, in cases where a semiconductor laser and light emission diode are used as the light emission device, such device changes in light emitting efficiency depending on temperature at the junctions. Namely, the light emitting efficiency is lowered as the temperature at the junction becomes higher.
The temperature at the junctions changes due to a duty ratio of a video signal or a drive current.
Accordingly, a large duty ratio of a video signal in the prior art has generated a problem because an exposure energy in the ending side of the scanning line is lowered by about 10 to 20% in comparison with that in the starting side of the scanning line.